In recent years, color liquid crystal display devices having various types of signal processing circuits have been developed in order to improve image quality, particularly for a liquid crystal television.
For example, Japanese Unexamined Patent Publication No. 10-240198/1998 (Tokukaihei 10-240198 published on Sep. 11, 1998) discloses a color liquid crystal display device including a chromaticity adjustment circuit. When one or two of signals of a red image display digital signal (R signal), a green image display digital signal (G signal) and a blue image display digital signal (B signal) have different gradation values, the chromaticity adjustment circuit adds a signal (signals) of an arbitrary value not less than 1 to the one or two of signals having smaller gradation values.
An object of the invention of the foregoing publication is to adjust a color reproduction range without using different color filters.
However, in the invention of Japanese Unexamined Patent Publication No. 240198/1998, since the chromaticity adjustment circuit raises gradation values of the one or two signals having smaller gradation values, the color reproduction range (chromaticity range) of chromatic pixels is reduced as the chromaticity coordinates points of chromatic pixels move to a white side as shown in FIG. 3 and FIG. 7 of the foregoing publication. Thus, chromaticity of the chromatic pixels is shifted closer to the white side, and chroma is decreased.
Further, the invention of the Japanese Unexamined Patent Publication No. 240198/1998 made it possible to adjust the color reproduction range; however, in a color liquid crystal display device, it is not possible to widen the color reproduction range of primary colors by controlling signal processing. This is because it is not possible to obtain luminance of more than 100% with respect to each pixel by controlling signal processing. In contrast, it is possible to reduce the color reproduction range, as shown in FIG. 3 and FIG. 7 of the foregoing publication. Namely, the invention of the Japanese Unexamined Patent Publication No. 240198/1998 only made it possible to correct the deviated white balance due to irregularities of color filters.
Meanwhile, in a shutter type color display such as a liquid crystal display, generally, light is not fully shaded by a light shutter, and chroma (color purity) is excessively reduced at a dark gradation because of the leakage of light.
Namely, for example, a contrast of a color liquid crystal display is about 300 in a general transmission type, and about 10 in a general reflection type. Accordingly, luminance of black display is about 1/10 or 1/300 when luminance of white display is 1, namely, black display of the color liquid crystal display device is not pure black display (luminance 0). Further, since external light reflects/scatters on a display screen of the color liquid crystal display in actual use environment, luminance of black display tends to further increase (the contrast tends to further decreases). Consequently, chroma of an image displayed in this general color liquid crystal display is low (the color is faded) compared to an ideal image display (an original image before it is reproduced, in the case of a television). In other words, the chromaticity coordinates points move to the white side. Further, many of the color liquid crystal display devices display a color image by separating colors with color filters. In this type of display devices, transmittance conflicts with color purity. Namely, for example, the use of highly colored filters improves chroma and color reproduction (color is enhanced) but decreases transmittance. On the other hand, in order to increase transmittance for higher luminance and lower power consumption (in the case of having a light source), it is necessary to use lightly colored filters, and therefore chroma (color purity) will decrease.
Accordingly, it has revealed that there has been a problem of a decrease in chroma (fading of color) in the color liquid crystal display device using color filters, which is caused by two facts:
(1) black display is not pure black display,
(2) color purity decreases when transmittance is increased.
Further, a projection type color display, a color electroluminescence display, a color plasma display and a color field emission display, which are lately receiving much public attention, also have the problem of a decrease in chroma.
Namely, chroma decreases due to reflection and scattering of external light especially in the case of dark display, in a color display having a reflective member or a diffuse transmission member on the display screen or in the vicinity of the display screen, such as a front projection type color display which projects a light beam toward a front surface of a reflection screen, a back projection type color display which projects a light beam from the back of a diffuse transmission screen, and a color electroluminescence display which has a reflective electrode in the vicinity of the back of a fluorescence layer.
In the case of a back projection type color display, the external light is scattered on the screen and a part of the light can be seen by an observer even in the case of black display since the display screen (projection screen) itself is made of a diffusing member, thereby decreasing the contrast. Further, in the case of a front projection type color display, the external light is reflected on the screen and the contrast decreases even in the case of black display since the display screen itself is made of a reflective member, thereby decreasing chroma (color purity).
Note that, in the projection type display, the black display is a dark screen normally recognized as a white screen but having no projection light thereon, and a white screen is a screen in which light is projected to the foregoing screen.
Further, in the electroluminescence display having a back electrode in the vicinity of the back of a fluorescence layer, this back electrode is often made of a reflective electrode such as aluminum. In the color electroluminescence display having the reflective electrode as the back electrode, the external light is reflected on the back electrode and the contrast decreases even in the case of black display, thereby decreasing chroma (color purity).
Further, a display adopting an excited fluorescence body such as a plasma display or a field emission display also have a problem of decrease in luminance with increase in color purity of the fluorescence body. Namely, in order to ensure high luminance of a display, a fluorescence body having low color purity and high luminance should be adopted. However, in this display, chroma will decrease. Accordingly, in order to realize a display having desirable color reproduction by adopting the fluorescence body having low color purity and high luminance, chroma is required to be increased (enhanced). As described, a problem of decrease in chroma is found in (1) a shutter type color display such as a color liquid crystal display, (2) a projection type display or an electroluminescence display having a reflective member or a diffuse transmission member in the display screen or in the vicinity of the display screen, (3) a color plasma display, and (4) a color field emission display.
Therefore, the chromaticity adjustment circuit of the Publication No. 240198/1998 (Tokukaihei 10-240198) will further decrease chroma in those displays against the original purpose.